Weatherwatch: The 'Pineapple express' flows from Hawaii to the US

Under the elm leaf-shape structure are cooling condensers that soak up humidity from the desert air. Photograph: Ap Verheggen/AP

When you watch the clouds rolling by overhead, you're seeing thousands of tons of water in rapid motion, and there may be an equal amount present invisible as humidity. Sometimes this movement is great enough to constitute an "atmospheric river", an expression coined by MIT meteorologists Reginald Newell and Yong Zhu in 1994.

An atmospheric river is a corridor of very moist air a few hundred kilometres wide and several thousand kilometres long. According to Zhu and Young there are typically six to 10 of them around the world at any one time. They are not fixed but move with weather systems. A large one can transport more water than the Amazon, the biggest terrestrial river in the world. Atmospheric rivers may stall and release their load of water when they reach land, making them major contributors to rainfall.

Not surprisingly, they are also behind some periods of extreme rainfall and heavy flooding in middle latitudes, including much of Europe and North America. A recurring atmospheric river nicknamed "Pineapple Express" brings a large mass of moist air from Hawaii to the mainland US, and produced major flooding in 2005 and 2010. A recent study by the University of Reading suggests that all major autumn and winter flooding in the UK over the last 40 years can be traced directly to atmospheric rivers. A better understanding of the dynamics of these rivers in the sky could help us assess and deal with flood risk.